Lithographic imaging with reduced power requirements

ABSTRACT

Imaging of lithographic printing plates with reduced fluence requirements is accomplished using printing members that have a solid substrate, gas-producing and radiation-absorptive layers over the substrate, and a topmost layer that contrasts with the substrate in terms of lithographic affinity. Exposure of the radiation-absorptive layer to laser light causes this layer to become intensely hot. This, in turn, activates the gas-producing layer, causing rapid evolution and expansion of gaseous decomposition products. The gases stretch the overlying topmost layer to create a bubble over the exposure region, where the imaging layers have been destroyed. If this process is sufficiently explosive, the neck of the bubble expands beyond the diameter of the incident laser beam, tearing the topmost layer and the underlying imaging layers away from the substrate outside the exposed region. The entire affected area is easily removed during a post-imaging cleaning process, resulting in an image spot larger than the incident beam diameter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to digital printing methods and materials,and more particularly to imaging of lithographic printing-plateconstructions on- or off-press using digitally controlled laser output.

2. Description of the Related Art

In offset lithography, a printable image is present on a printing memberas a pattern of ink-accepting (oleophilic) and ink-rejecting(oleophobic) surface areas. Once applied to these areas, ink can beefficiently transferred to a recording medium in the imagewise patternwith substantial fidelity. Dry printing systems utilize printing memberswhose ink-repellent portions are sufficiently phobic to ink as to permitits direct application. Ink applied uniformly to the printing member istransferred to the recording medium only in the imagewise pattern.Ordinarily, the printing member first makes contact with a compliantintermediate surface called a blanket cylinder which, in turn, appliesthe image to the paper or other recording medium. In typical sheet-fedpress systems, the recording medium is pinned to an impression cylinder,which brings it into contact with the blanket cylinder.

In a wet lithographic system, the non-image areas are hydrophilic, andthe necessary ink-repellency is provided by an initial application of adampening (or “fountain”) solution to the plate prior to inking. Thefountain solution prevents ink from adhering to the non-image areas, butdoes not affect the oleophilic character of the image areas.

Traditional platemaking processes tend to be time-consuming and requirefacilities and equipment adequate to support the necessary chemistry. Tocircumvent these shortcomings, practitioners have developed a number ofelectronic alternatives to plate imaging. With these systems, digitallycontrolled devices alter the ink-receptivity of blank plates in apattern representative of the image to be printed. U.S. Pat. Nos.5,339,737, 5,783,364, and 5,807,658, the entire disclosures of which arehereby incorporated by reference, disclose a variety of lithographicplate configurations for use with imaging apparatus that operate bylaser discharge. These include wet plates as described above and dryplates to which ink is applied directly. The plates may be imaged on astand-alone platemaker or directly on-press.

Laser-imageable materials may be imaged by pulses of near-infrared(near-IR) light from inexpensive solid-state lasers. Such materialstypically exhibit a nonlinear response to near-IR exposure, namely, arelatively sharp imaging-fluence threshold for short-duration laserpulses but essentially no response to ambient light. A longstanding goalof plate designers is to reduce the threshold laser fluence necessary toproduce an imaging response while maintaining desirable properties suchas durability, manufacturability, and internal compatibility.

One strategy frequently proposed in connection with photothermalmaterials is incorporation of energetic (e.g., self-oxidizing)compositions, which, in effect, contribute chemical energy to theimaging process. For example, the '737 patent mentioned above disclosesnitrocellulose layers that undergo energetic chemical decomposition inresponse to heating. Unfortunately, these materials have not been shownto reduce the fluence thresholds necessary for imaging. Instead, theyare either employed as essentially interchangeable alternatives tonon-energetic materials, or as propellant layers in transfer-typematerials (see, e.g., U.S. Pat. Nos. 5,308,737, 5,278,023, 5,156,938,and 5,171,650).

DESCRIPTION OF THE INVENTION

Brief Summary of the Invention

It has been found, surprisingly, that the combination of a thermallyactivated gas-forming composition with a material that strongly absorbsimaging radiation, used as co-active imaging layers in a laser-imageableconstruction, results in substantial enlargement of the area affected bya laser pulse (as compared with constructions utilizing as imaginglayers either component alone). The result is considerable reduction inthe fluence necessary to create an image spot of a given size.

A printing member in accordance with the present invention includes asolid substrate, gas-producing and radiation-absorptive layers over thesubstrate, and a topmost layer that contrasts with the substrate interms of lithographic affinity. The order in which the gas-producing andradiation-absorptive layers appear depends on the mode of imaging—thatis, whether laser radiation is applied through the topmost layer orthrough the substrate. In operation, exposure of theradiation-absorptive layer to laser light causes this layer to becomeintensely hot. This, in turn, activates the gas-producing layer, causingrapid evolution and expansion of gaseous decomposition products. Thegases stretch the overlying topmost layer to create a bubble over theexposure region, where the imaging layers have been destroyed. If thisprocess is sufficiently explosive, the neck of the bubble expands beyondthe diameter of the incident laser beam, tearing the topmost layer andthe underlying imaging layers away from the substrate outside theexposed region. The entire affected area is easily removed during apost-imaging cleaning process, resulting in an image spot larger thanthe incident beam diameter. Furthermore, because the decomposition gasesare retained within the bubble, there is no danger of environmentalcontamination.

Post-imaging cleaning can be accomplished either manually (by dryrubbing or rubbing with a cleaning liquid, as described in U.S. Pat. No.5,540,150) or using a contact cleaning device (e.g., a rotating brush asdescribed in U.S. Pat. No. 5,148,746) or other suitable means (e.g., asset forth in U.S. Pat. No. 5,755,158).

It should be stressed that, as used herein, the term “plate” or “member”refers to any type of printing member or surface capable of recording animage defined by regions exhibiting differential affinities for inkand/or dampening fluid; suitable configurations include the traditionalplanar or curved lithographic plates that are mounted on the platecylinder of a printing press, but can also include seamless cylinders(e.g., the roll surface of a plate cylinder), an endless belt, or otherarrangement.

Furthermore, the term “hydrophilic” is herein used in the printing senseto connote a surface affinity for a fluid which prevents ink fromadhering thereto. Such fluids include water, aqueous and non-aqueousdampening liquids, the non-ink phase of single-fluid ink systems. Thus,a hydrophilic surface in accordance herewith exhibits preferentialaffinity for any of these materials relative to oil-based materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying drawings, in which:

FIG. 1 is an enlarged sectional view of a lithographic plate imageablein accordance with the present invention; and

FIGS. 2A-2C illustrate the imaging process of the present invention interms of its effects on the plate illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a representative embodiment of a printingplate in accordance with the invention includes a topmost layer 10, aradiation-absorptive layer 12, a gas-producing layer 14, and a substrate20. Layers 10 and 20 exhibit opposite affinities for ink or fluid towhich ink will not adhere, and generally, layer 10 will be polymeric. Inone version of this plate, topmost layer 10 is a silicone polymer thatrepels ink, while substrate 20 is an oleophilic polyester or aluminummaterial; the result is a dry plate. In a second, wet-plate version,surface layer 10 is a hydrophilic material while substrate 20 is botholeophilic and hydrophobic.

Preferred silicone formulations are addition-cure polysiloxanes, such asthose described in U.S. Pat. No. Re. 35,512, the entire disclosure ofwhich is hereby incorporated by reference; suitable hydrophilic polymersinclude polyvinyl alcohol materials (e.g., the Airvol 125 materialsupplied by Air Products, Allentown, Pa.).

Substrate 20 is preferably strong, stable and flexible, and may be apolymer film, or a paper or metal sheet. Polyester films (in a preferredembodiment, the MYLAR or MELINEX films sold by E.I. duPont de NemoursCo., Wilmington, Del.) furnish useful examples. A preferredpolyester-film thickness is 0.007 inch, but thinner and thicker versionscan be used effectively. Paper substrates are typically “saturated” withpolymerics to impart water resistance, dimensional stability andstrength. Aluminum is a preferred metal substrate.

Depending on the thicknesses and optical densities of imaging layers 12,14, the substrate (or a layer thereunder) may be reflective of imagingradiation so as to redirect it back into the imaging layers. Forexample, an aluminum substrate 20 may be polished to reflect imagingradiation. One can also employ, as an alternative to a metal reflectivesubstrate 20, a layer containing a pigment that reflects imaging (e.g.,IR) radiation. A material suitable for use as an IR-reflective substrateis the white 329 film supplied by ICI Films, Wilmington, Del., whichutilizes IR-reflective barium sulfate as the white pigment. A preferredthickness is 0.007 inch, or 0.002 inch if the construction is laminatedonto a metal support (as described, for example, in the '512 patent).

In accordance with copending application Ser. No. 09/122,261 (filed onJul. 24, 1998 and entitled METHOD OF LITHOGRAPHIC IMAGING WITH REDUCEDDEBRIS-GENERATED PERFORMANCE DEGRADATION AND RELATED CONSTRUCTIONS), theentire disclosure of which is hereby incorporated by reference, it ispossible to add an insulating (e.g., polysilane) layer between topmostlayer 10 and layers 12, 14 for purposes of debris management. For thesame purpose, one may disperse a solid filler material such asparticulate silica within topmost layer 10 in order to generate debriswith hydrophilic sites, rendering them compatible with cleaningsolutions.

Layer 12 may be a very thin (50-500 Å, with 250 Å preferred fortitanium) layer of a metal that may or may not develop a native oxidesurface 12 s upon exposure to air. This layer ablates in response to IRradiation, undergoing catastrophic overheating and thereby ignitinglayer 14. Although the preferred material is titanium, other materialssuitable for layer 12 include other d-block (transition) metals,aluminum, indium, tin, silicon, and bismuth, either singly or incombination. In the case of a mixture, the metals are present as analloy or an intermetallic.

An alternative material, which may be used in conjunction with or inlieu of a metal layer 12 as described above, is a metallic inorganiclayer comprising a compound of at least one metal with at least onenon-metal, or a mixture of such compounds. Such a layer is generallyapplied at a thickness of 50-500 Å; optimal thickness is determinedprimarily the need for rapid heating to a very high temperature uponabsorption of laser energy, but also by functional concerns—i.e., theneed for intercoat adhesion and resistance to the effects of fluids usedin the printing process. The metal component of a suitable metallicinorganic layer may be a d-block (transition) metal, an f-block(lanthanide) metal, aluminum, indium or tin, or a mixture of any of theforegoing (an alloy or, in cases in which a more definite compositionexists, an intermetallic). Preferred metals include titanium, zirconium,vanadium, niobium, tantalum, molybdenum and tungsten. The non-metalcomponent may be one or more of the p-block elements boron, carbon,nitrogen, oxygen and silicon. A metal/non-metal compound in accordanceherewith may or may not have a definite stoichiometry, and may in somecases (e.g., Al—Si compounds) be an alloy. Preferred metal/non-metalcombinations include TiN, TiON, TiO_(x) (where 0.9≦x≦2.0), TiC, andTiCN.

Layer 14 comprises or constitutes a material that evolves gas (e.g., N₂)upon rapid heating. Heat-responsive polymers that liberate nitrogen gastypically contain thermally decomposable functional groups. The polymermay itself be gas-liberating or may instead contain a decomposablematerial (e.g., diazonium salts or another polymer) dispersed orotherwise integrated within the polymer matrix. Thermally decomposablefunctional groups include azo, azide, and nitro; see, e.g., U.S. Pat.Nos. 5,308,737 and 5,278,023. The thermally decomposable groups may beincorporated into the gas-producing polymer either prior topolymerization or by modification of an existing polymer (e.g., bydiazotization of an aromatic ring with sodium nitrite, or diazo transferwith tosyl azide onto an amine or β-diketone in the presence oftriethylamine).

The gas-producing material may be an “energetic polymer,” defined hereinas a polymer containing functional groups that exothermically decomposeto generate gases under pressure when rapidly heated (generally on atime scale ranging from nanoseconds to milliseconds) above a thresholdtemperature.

Such polymers may contain, for example, azido, nitrato, and/or nitraminofunctional groups. Examples of energetic polymers includepoly[bis(azidomethyl)]oxetane (BAMO), glycidyl azide polymer (GAP),azidomethyl methyloxetane (AMMO), polyvinyl nitrate (PVN),nitrocellulose, acrylics, and polycarbonates.

The material of layer 14 may include a compound sensitive to (i.e.,absorptive of) the imaging radiation. This allows radiation passingthrough layer 12 (or the remainder of the imaging pulse followingablation of layer 12) to contribute to heating of layer 14. For example,in the case of IR imaging, an IR-absorptive dye (e.g., the Kodak IR-810dye available from Eastman Fine Chemicals, Eastman Kodak Co., Rochester,N.Y.) or pigment (e.g., the Heliogen Green L 8730 green pigment suppliedby BASF Corp., Chemicals Division, Holland, Mich.) may be employed toadvantage.

Imaging apparatus suitable for use in conjunction with the presentprinting members includes at least one laser device that emits in theregion of maximum plate responsiveness, i.e., whose lambda_(max) closelyapproximates the wavelength region where layer 12 absorbs most strongly.The device may be a diode laser or, for greater speed, a Q-switched YAGlaser. Specifications for diode lasers that emit in the near-IR regionare fully described in the '512 patent and in U.S. Pat. Nos. 5,385,092,5,822,345, 4,577,932, 5,517,359, 5,802,034, 5,475,416, and 5,521,748(the entire disclosures of which are hereby incorporated by reference);see also published European Patent Application No. 0601485. YAG lasersand lasers emitting in other regions of the electromagnetic spectrum arewell-known to those skilled in the art.

The thickness of layer 14 naturally depends on the material selected.Generally, however, the thickness will be on the order of 0.5-3 μm.

Suitable imaging configurations are also set forth in detail in the'512, '092, '345 and other patents mentioned above. Briefly, laseroutput can be provided directly to the plate surface via lenses or otherbeam-guiding components, or transmitted to the surface of a blankprinting plate from a remotely sited laser using a fiber-optic cable. Acontroller and associated positioning hardware maintains the beam outputat a precise orientation with respect to the plate surface, scans theoutput over the surface, and activates the laser at positions adjacentselected points or areas of the plate. The controller responds toincoming image signals corresponding to the original document or picturebeing copied onto the plate to produce a precise negative or positiveimage of that original. The image signals are stored as a bitmap datafile on a computer. Such files may be generated by a raster imageprocessor (RIP) or other suitable means. For example, a RIP can acceptinput data in page-description language, which defines all of thefeatures required to be transferred onto the printing plate, or as acombination of page-description language and one or more image datafiles. The bitmaps are constructed to define the hue of the color aswell as screen frequencies and angles.

The imaging apparatus can operate on its own, functioning solely as aplatemaker, or can be incorporated directly into a lithographic printingpress. In the latter case, printing may commence immediately afterapplication of the image to a blank plate, thereby reducing press set-uptime considerably. The imaging apparatus can be configured as a flatbedrecorder or as a drum recorder, with the lithographic plate blankmounted to the interior or exterior cylindrical surface of the drum.Obviously, the exterior drum design is more appropriate to use in situ,on a lithographic press, in which case the print cylinder itselfconstitutes the drum component of the recorder or plotter.

In the drum configuration, the requisite relative motion between thelaser beam and the plate is achieved by rotating the drum (and the platemounted thereon) about its axis and moving the beam parallel to therotation axis, thereby scanning the plate circumferentially so the image“grows” in the axial direction. Alternatively, the beam can moveparallel to the drum axis and, after each pass across the plate,increment angularly so that the image on the plate “grows”circumferentially. In both cases, after a complete scan by the beam, animage corresponding (positively or negatively) to the original documentor picture will have been applied to the surface of the plate.

In the flatbed configuration, the beam is drawn across either axis ofthe plate, and is indexed along the other axis after each pass. Ofcourse, the requisite relative motion between the beam and the plate maybe produced by movement of the plate rather than (or in addition to)movement of the beam.

Regardless of the manner in which the beam is scanned, it is generallypreferable (for on-press applications) to employ a plurality of lasersand guide their outputs to a single writing array. The writing array isthen indexed, after completion of each pass across or along the plate, adistance determined by the number of beams emanating from the array, andby the desired resolution (i.e., the number of image points per unitlength). Off-press applications, which can be designed to accommodatevery rapid plate movement (e.g., through use of high-speed motors) andthereby utilize high laser pulse rates, can frequently utilize a singlelaser as an imaging source.

FIGS. 2A-2C illustrate the mode of operation of the present invention.Laser output is directed through layer 10; accordingly, absorptive layer12 overlies gas-producing layer 14. YAG lasers emits “single-mode”radiation—that is, a beam having a radially symmetric Gaussian energydistribution. The bulk of the beam's energy is concentrated in a single,central peak, and falls off radially and smoothly in all directionsaccording to the Gaussian function. A single-mode laser pulse is shownat 50, with the arrows indicating the radial energy distribution. Adiode laser, by contrast, emits a “top hat” energy profile with sharpfalloff occurring at the beam periphery. The invention may be practicedwith virtually any laser profile, although the Gaussian YAG profile,with its centrally concentrated beam energy, contributes to the abilityto image with shorter-duration pulses.

In either case, the imaging pulse strikes layer 12, causing that layerto absorb energy and effect rapid heating of underlying layer 14. Layer14, in turn, generates gas-phase thermal decomposition products that aretrapped beneath topmost layer 10. Layer 10 is elastic; as a result, abubble 60 is formed (see FIG. 2B). The neck or base of the bubble is inthe plane of substrate 10, and layers 12, 14 substantially ablate withinthe initial diameter d of the bubble (which matches the diameter of theincident laser beam 50).

If layer 14 releases a sufficient volume of gas under enough pressure,the neck of bubble 60 will expand beyond the exposure region d,overcoming the forces of adhesion between layer 14 and substrate 20. Theaffected area has a diameter d′>d, and the de-anchored portions oflayers 12, 14 are removed along with the overlying layer 10 bypost-image cleaning. Consequently, the resulting image spot has adiameter greater than that of the incident laser beam.

Not surprisingly, it is found experimentally that the increase in thearea of the image over the area of the incident beam depends strongly onthe material of layer 14. For example, with a silicone layer 10 and anacrylic polymer layer 14, application by a YAG laser of a 110 nsec laserpulse having an energy of 10 μJ creates an image spot with an area 50%larger than that obtained on constructions omitting layer 14.

Substituting a more energetic nitrocellulose layer 14, the area of theresulting image spot is observed to be more than 100% larger. Using adiode laser, a 4 μsec pulse applied to a construction having a siliconelayer 10 and a nitrocellulose layer 14 creates a 50% increase in imagespot size.

The effect also depends on the duration of the imaging pulse. Energymust be delivered quickly in order to create a response. Very longpulses (i.e., durations in excess of 30 μsec) fail to concentratesufficient heat to cause any imaging effect due to heat-sinking anddispersive effects; it is for this reason that laser-imageable plates inaccordance herewith do not undergo spontaneous response in ambientlight. An exposure duration on the order of 10 μsec melts the metallayer 12 and causes it to recede radially, producing an image spot uponsubsequent cleaning, but the image spot is actually smaller than theincident beam diameter. It is found that exposure durations on the orderof 5 μsec or less create the desired effect, i.e., an image spot largerthan the effective beam area. These durations can be obtained usingdiode laser or YAG systems, although the latter are currently capable ofmuch shorter-duration (i.e., nsec range) pulses due to higher outputpower; shorter-duration pulses, even with less total energy delivered,can result in greater degrees of enlargement due to the reducedopportunity for heat dissipation.

It will therefore be seen that the foregoing represents a highlyadvantageous approach to laser recording, facilitating reliable imagingwith reduced laser fluence requirements. The terms and expressionsemployed herein are used as terms of description and not of limitation,and there is no intention, in the use of such terms and expressions, ofexcluding any equivalents of the features shown and described orportions thereof, but it is recognized that various modifications arepossible within the scope of the invention claimed.

What is claimed is:
 1. A method of imaging a lithographic printingmember, the method comprising the steps of: a. providing a printingmember having (i) a solid substrate, (ii) first and second imaginglayers over the substrate, and (iii) a topmost layer, the topmost layerand the substrate having different affinities for at least one of inkand a fluid to which ink will not adhere, the first imaging layercomprising a thermally activated gas-forming composition but notincluding a material absorptive of imaging radiation and the secondlayer comprising a material absorptive of imaging radiation, the secondlayer becoming sufficiently hot, upon absorption of said radiation, tocause evolution of gas from the first layer; b. selectively exposing, ina pattern representing an image, the printing member, whereby the firstand second imaging layers are destroyed and the topmost layer detachedby the evolved gas in the exposed region; and c. removing remnants ofthe first layer where the printing member received radiation, therebyrevealing the substrate to form a lithographic image.
 2. The method ofclaim 1 wherein the exposure step comprises subjecting the printingmember to a laser beam in accordance with the pattern, the laser beamhaving a beam diameter, each exposure to the laser beam causingdestruction of the first and second layers and detachment of the topmostlayer over an area larger than the beam diameter.
 3. The method of claim1 wherein the topmost layer is oleophobic and the substrate isoleophilic.
 4. The method of claim 3 wherein the topmost layer issilicone.
 5. The method of claim 1 wherein the topmost layer ishydrophilic and the substrate is oleophilic.
 6. The method of claim 5wherein the topmost layer is polyvinyl alcohol.
 7. The method of claim 1wherein the first layer is an energetic polymer and the second layer isa metal.
 8. The method of claim 7 wherein the metal layer comprises atleast one of (i) a d-block transition metal, (ii) aluminum, (iii) indiumand (iv) tin.
 9. The method of claim 8 wherein the metal is titanium.10. The method of claim 7 wherein the energetic polymer comprises atleast one functional group selected from azo, azide, and nitro.
 11. Themethod of claim 7 wherein the energetic polymer is selected from thegroup consisting of poly[bis(azidomethyl)]oxetane, glycidyl azidepolymer, azidomethyl methyloxetane, polyvinyl nitrate, nitrocellulose,acrylics, and polycarbonates.
 12. The method of claim 1 wherein thefirst layer is an energetic polymer and the second layer comprises ametallic inorganic compound comprising a metal and a non-metal.
 13. Themethod of claim 1 wherein the first layer overlies the substrate and thesecond layer overlies the first layer, the topmost layer issubstantially transparent to imaging radiation, and imaging radiation isapplied through the topmost layer.
 14. The method of claim 13 whereinthe first layer includes a material sensitive to imaging radiation. 15.The method of claim 1 wherein the first layer underlies the substrateand the second layer overlies the first layer, the substrate issubstantially transparent to imaging radiation, and imaging radiation isapplied through the substrate.
 16. The method of claim 1 wherein theexposure step comprises subjecting the printing member to a laser beamin accordance with the pattern, the laser beam having a pulse durationno greater than 5 μsec.
 17. A lithographic printing member comprising:a. a solid substrate; b. first and second imaging layers over thesubstrate; and c. a topmost layer, the topmost layer and the substratehaving different affinities for at least one of ink and a fluid to whichink will not adhere, the first imaging layer comprising a thermallyactivated gas-forming composition but not including a materialabsorptive of imaging radiation and the second layer comprising amaterial absorptive of imaging radiation, the second layer becomingsufficiently hot, upon absorption of said radiation, to cause evolutionof gas from the first layer.
 18. The member of claim 17 wherein thetopmost layer is oleophobic and the substrate is oleophilic.
 19. Themember of claim 18 wherein the topmost layer is silicone.
 20. The memberof claim 17 wherein the topmost layer is hydrophilic and the substrateis oleophilic.
 21. The member of claim 20 wherein the topmost layer ispolyvinyl alcohol.
 22. The member of claim 17 wherein the first layer isan energetic polymer and the second layer is a metal.
 23. The member ofclaim 22 wherein the metal layer comprises at least one of (i) a d-blocktransition metal, (ii) aluminum, (iii) indium and (iv) tin.
 24. Themember of claim 23 wherein the metal is titanium.
 25. The member ofclaim 17 wherein the first layer is an energetic polymer and the secondlayer comprises a metallic inorganic compound comprising a metal and anon-metal.
 26. The member of claim 25 wherein the energetic polymercomprises at least one functional group selected from azo, azide, andnitro.
 27. The member of claim 25 wherein the energetic polymer isselected from the group consisting of poly[bis(azidomethyl)]oxetane,glycidyl azide polymer, azidomethyl methyloxetane, polyvinyl nitrate,nitrocellulose, acrylics, and polycarbonates.
 28. The member of claim 17wherein the first layer overlies the substrate and the second layeroverlies the first layer.
 29. The member of claim 17 wherein the firstlayer underlies the substrate and the second layer overlies the firstlayer.